The present invention relates to all of the mass spectrometers including a quadrupole ion trap process, such as a quadrupole ion trap mass spectrometer and a quadrupole-ion-trap/time-of-flight mass spectrometer.
As one example of a variety of mass analyzing methods, there exist ion trap mass analyzing methods. The basic principle of a quadrupole ion trap mass analyzing method has been described in U.S. Pat. No. 4,650,999. In the ion trap scheme, an about 1-MHz radio frequency voltage is applied to a ring electrode so as to accumulate ions. Within an ion trap, ions whose mass numbers are larger than a certain value acquire a stabilizing condition, thereby being accumulated. After that, the ring voltage is swept from the lower value to a higher one. At this time, the trapped ions are sequentially ejected from an ion with the smallest mass number. This makes it possible to obtain the mass spectrum. The scheme described in U.S. Pat. No. 4,650,999 however, finds it impossible to differentiate different types of ions whose mass numbers are identical to each other.
In order to improve this drawback, a tandem mass spectrometry in the ion trap has been developed. As one example of the tandem mass spectrometry in the quadrupole ion trap, there exists a collision-induced dissociation method based on the collisions with a bath gas within the quadrupole ion trap. This scheme has been described in U.S. Pat. No. 4,736,101. In the present scheme, ions generated at an ion source are accumulated within the ion trap, then isolating parent ions that have a desired mass number. After the ion isolation, a supplemental AC electric field that resonates with the parent ions is applied between endcap electrodes, thereby enlarging the ion orbits. This causes the ions to collide with the neutral gas filling the ion trap, thereby dissociating and detecting the ions. The resultant product ions exhibit specific patterns attributed to differences in the molecular structures. Accordingly, it becomes possible to differentiate the different types of ions whose mass numbers are identical to each other. In order to dissociate the ions, however, it is necessary to increase the ion trapping potential generated by the ring voltage. In order to increase the ion trapping potential, in turn, it is necessary to set up the ring voltage to a high-voltage. This gives rise to a problem that the product ions with small mass numbers deviate from the stable orbit condition and become incapable of being trapped.
In order to solve the above-described problem in the collision-induced dissociation, a method of performing the dissociation with the use of infrared laser has been disclosed in xe2x80x9cAnalytical Chemistryxe2x80x9d 1996, Vol. 68, page 4033. According to this method, after the ion isolation, an irradiation with CO2 laser is performed from a hole, which is bored in the ring electrode, toward the ion trap""s central region. The absorption of the infrared laser light by the ions excites the internal energies, which develops the dissociation of the ions. The present scheme allows the small mass-number product ions to be detected by the quadrupole ion trap mass spectrometer. Boring the hole in the ring electrode, however, disturbs a quadrupole electric field within the ion trap, thereby deteriorating the sensitivity and the resolution. Also, the bath-gas pressure (lower than 0.1 mTorr) within the ion trap, which is needed when using an about 50-W output CO2 laser, does not coincide with the optimum degree of vacuum (about 1 to 3 mTorr) for maintaining the ion trapping efficiency and sensitivity. On account of this, the conventional dissociation using the laser light has found it impossible to perform the ion accumulation and dissociation in the ion trap at the optimum degree of vacuum. Consequently, in the conventional ion trap mass spectrometers using the laser light, there has existed a problem that the ion trapping efficiency and sensitivity are considerably low.
Also, a method of performing the infrared laser irradiation and the application of the supplemental AC voltage between the endcap electrodes has been disclosed in xe2x80x9cAnalytical Chemistryxe2x80x9d 2001, Vol. 73, page 1270. According to this method, the collision-induced dissociation by the application of the supplemental AC voltage and the infrared multiphoton dissociation by the infrared laser irradiation are performed at different points-in-time subsequently to each other. This makes it possible to obtain product ions specific to the respective dissociation methods, thus resulting in an advantage of being able to obtain the complementary information.
Also, a method of simultaneously performing the infrared laser irradiation and the application of the supplemental AC voltage between the endcap electrodes has been disclosed in xe2x80x9cAnalytical Chemistryxe2x80x9d 2001, Vol. 73, page 3542. According to this method, the incident direction of the laser light and the application direction of the resonance voltage are located perpendicularly to each other. The supplemental AC electric field is applied between the endcap electrodes, thereby enlarging a desired ion orbit. This shortens a time-period during which the ion whose orbit has been spread by the resonance will undergo the laser irradiation. In this case, the ion whose orbit has been spread exhibits an effect of suppressing the dissociation. Accordingly, it becomes possible to suppress, in the isolated manner, the dissociation of ions included in a particular mass-number range.
It is an object of the present invention to provide an ion trap mass spectrometer that allows small mass-number product ions to be detected without damaging the sensitivity and the resolution.
In the mass spectrometer according to the present invention, ions are accumulated within the ion trap. Moreover, light and an AC electric field are applied to the accumulated ions, thereby dissociating the ions. At that time, the direction of the AC electric-field vector to be applied to the ions in a supplemental manner in order to dissociate them and the application direction of the light to be applied thereto in order to dissociate them are made identical to each other. As compared with the prior arts, the present mass spectrometer makes it possible to detect the small mass-number product ions with a higher-efficiency. This, eventually, increases the information amount made available by the present mass spectrometer, thereby enhancing the quality-analysis capabilities and the quantity-analysis capabilities.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.